Representative embodiments are directed to vacuum pump systems for evacuating enclosed chambers of devices or apparatus, such as processing chambers. Representative embodiments are also directed to leak detection apparatus including vacuum pump systems.
There are various industrial applications in which gases of low molecular weight, e.g., helium or hydrogen, must be pumped into or from an enclosed chamber. An example of such an application is gas chromatography in which helium or hydrogen used as a carrier gas for a sample analyte is pumped into a mass spectrometer. Another application is leak detection in which a gas of low molecular weight is provided in the ambient atmosphere around a chamber to be tested for leaks (test object), and gas in the chamber is pumped from the chamber and into a leak detection sensor capable of sensing the gas of low molecular weight. In these types of applications a vacuum pumping system is used to create a vacuum that draws gas from and/or induces gas into an enclosed chamber. One type of pump that is used in vacuum pumping systems for pumping gases, including those of low molecular weight, is a scroll vacuum pump.
A scroll pump includes a stationary plate scroll having a spiral stationary scroll blade, an orbiting plate scroll having a spiral orbiting scroll blade, and an eccentric driving mechanism to which the orbiting plate scroll is coupled. The stationary and orbiting scroll blades are nested with a radial clearance and predetermined relative angular positioning such that a series of pockets, constituting a compression stage of the pump, are simultaneously defined by and between the blades. The orbiting plate scroll and hence, the orbiting scroll blade, is driven by the eccentric driving mechanism to orbit relative to the stationary plate scroll about a longitudinal axis of the pump passing through the axial center of the stationary scroll blade. As a result, the volumes of the pockets delimited by the scroll blades of the pump are varied as the orbiting scroll blade moves relative to the stationary scroll blade. The orbiting motion of the orbiting scroll blade also causes the pockets to move within the pump head assembly such that the pockets are selectively placed in open communication with an inlet and outlet of the scroll pump.
In a vacuum scroll pump, the motion of the orbiting scroll blade relative to the stationary scroll blade causes a pocket sealed off from the outlet of the pump and in open communication with the inlet of the pump to expand. Accordingly, fluid is drawn into the pocket through the inlet. The inlet of the pump is connected to a system that is to be evacuated, e.g., a system including a processing chamber in which a vacuum is to be created and/or from which gas is to be discharged. Then the pocket is moved to a position at which it is sealed off from the inlet of the pump and is in open communication with the outlet of the pump, and at the same time the pocket is contracted. Thus, the fluid in the pocket is compressed and thereby discharged through the outlet of the pump.
In the vacuum pump systems applied to gas chromatography, leak detection, and the like, scroll pumps possess the advantage of not using oil, which could otherwise contaminate the instrumentation and result in false readings. Furthermore, in most applications an exhaust check valve is provided over the outlet of the vacuum scroll pump to prevent a reverse flow of gas during certain portions of the compression cycle, which would degrade the efficiency of the vacuum pump. However, as described above, a vacuum scroll pump relies on very small clearances between the blades of the orbiting and stationary scroll blades to maintain seals in between the pockets created between the inlet and outlet of the pump. Leakage through these clearances may occur during operation especially before enough pressure is created in the downstream pocket to open the exhaust check valve. These clearances are small enough that leakage at the seals is negligible when pumping air or gases of similar molecular weight, i.e., loss due to gas leakage is acceptable. On the other hand, the small molecules of gases of low molecular weight pass relatively easily through the small clearances between the stationary and orbiting scroll blades and move upstream in the pump. Accordingly, vacuum scroll pumps may not be very efficient, at pumping gases of low molecular weight, in terms of volumetric pumping speed or compression ratio.
Moreover, vacuum scroll pumps are often used to remove air from chambers where the air may contain water vapor as a result of humidity. In this case, the water vapor in the air being exhausted may condense as the gas is compressed. The solid lines in the graph of FIG. 1 show the compression process as air is moved from the inlet to the outlet of the pump. In this case, the discharge port is that portion of the outlet just upstream of the exhaust check valve as normally closed from the outside by the valve head of the check valve. If the amount of water vapor in the gas is relatively large, the saturation temperature of the gas being a function of both pressure and temperature, the saturation temperature will eventually exceed the actual gas temperature, at which point water will form as condensate of the gas between the blades of the scroll pump. This water can corrode components of the pump, and can absorb gases being pumped which can cause problems in the operation of the pump, etc.
To prevent condensation of gas inside a vacuum scroll pump, additional gas (air or dry nitrogen, for example) is directed into the compression stage through a gas passageway at a location near but not at the downstream end of the compression stage; this process being referred to as “gas ballast”. The ballast gas dilutes the gas being worked by the vacuum scroll pump in the compression stage. The added gas load also increases the temperature of the gas. The combination of these two factors reduces saturation temperature of the gas stream below the actual gas temperature and condensation of water vapor is prevented. The changes to the patterns of internal pressure are shown by the chained lines in FIG. 1. It can be seen that now the saturation temperature line and the gas temperature line no longer intersect; thus, condensation of water will not occur. In addition, the use of gas ballast applies to the vapors of other substances which will take liquid form at the combinations of pressure and temperature that can exist within a vacuum pump, e.g., various organic solvents.